Add a missing implicit null check in the ARM codegen.

The code generated for object ArraySet on ARM used to
miss an implicit null check for the array when the assigned
value is `null`.  This has not been an actual issue so far,
as ArraySet instructions have never been using implicit null
checks.

Note: This CL comes without a regression test, as the code
path in question is not used (yet).

Change-Id: If3bc85e32802595e635513dfb83ccfcfd8f00d3d